The present invention relates to an image recording apparatus for use in copying machines, printers, and facsimiles, etc., and more particularly, to a type thereof in which charged toners pass through aperture electrodes to form a toner image onto an image receiving member on an opposing electrode.
A conventional image recording apparatus of this type is schematically shown in FIG. 1. The apparatus includes toner flow control means 203 having a control electrode formed with apertures 204, toner supplying means 201 for supplying toners to the toner flow control means 203, and an opposing electrode 206 for attracting toners 14 passing through apertures 204 of the toner flow control means onto an image receiving medium P such as a sheet.
For maintenance and inspection, the opposing electrode 206 is pivotally movably provided in a direction indicated by an arrow A by an arm 207 whose one end is connected to a pivot shaft 208 supported to an outer body frame (not shown). The opposing electrode 206 is of a cylindrical shape. In order to obtain high grade toner image onto the sheet P, the sheet P must be in intimate contact with the opposing electrode 206. To this effect, the sheet P is fed in a direction indicated by an arrow B with partly windingly contacting an outer peripheral cylindrical surface of the opposing electrode 206.
A gap of 0.5 mm is provided between the opposing electrode 206 and the toner flow control means 201, and high voltage such as about 1 KV is applied to the opposing electrode 206. The image receiving medium P passes through the minute gap for receiving the charged toners. The gap distance between the opposing electrode 206 and the toner flow control means 201 must be constant at all time during image recording operation. And a constant parallelism must be provided therebetween to provide the constant gap, otherwise, flying or moving loci of toners passing through the apertures 204 of the toner flow control means 203 are changed, to degrade the toner image. Further, if the gap becomes excessively small, puncture may occur between the toner flow control means 203 and the opposing electrode 206 due to the high voltage applied to the opposing electrode 206. As a result, the toner flow control means and its driving circuit may be destroyed, to render the apparatus inoperative.
However, in such a conventional image recording apparatus, the minute gap distance between the opposing electrode 206 and the toner flow control means 203 is easily changed if an outer frame body of the apparatus is vibrated. Further, since opposing electrode 206 is pivotally movably supported by the arm 207, the position of the opposing electrode 206 may be changed, so that the gap is also changed due to the force of the sheet P partly winding around the opposing electrode 206. That is, when the sheet P passes through the opposing electrode 206, the sheet P urges the opposing electrode 206 to move in a direction to change the distance between the opposing electrode 206 and the toner flow control means 203.
For example, if the sheet winding angle at upstream portion P1 of the sheet relative to the opposing electrode 206 is greater than that at downstream portion P2, the opposing electrode 206 is urged downwardly due to frictional force occurring between the sheet portion P1 and the outer surface of the opposing electrode 206. As a result, the distance of the gap is decreased, to provide a danger of the puncture.
It would be difficult to accurately support the opposing electrode 206 to the outer frame body so as to obtain the constant space relative to the toner flow control means 203 due to the requirement of high dimensional accuracy. Even if the opposing electrode 206 is directly supported to the outer frame body in an attempt to obviate the problem of relative vibration between the outer frame body and the opposing electrode, or in an attempt to avoid pivotal movement of the opposing electrode, the change in the gap distance may still occur due to minute deformation or bending of the outer frame body. Employment of extremely rigid outer frame body is costly.
Japanese Patent Application Kokai No. Hei 3-168769 discloses an image recording apparatus of this type as shown in FIGS. 2(a) and 2(b). This apparatus includes a flat control electrode 103 disposed or netted at a lower opening of a toner case 108 in which a toner carrier 101 is rotatably disposed. An opposing electrode 106 is upwardly biased by a spring 134 and is provided immediately below and in confrontation with the control electrode 103. An electrically insulative position regulating member 136 is provided at the bottom of the toner case 108 so as to regulate the vertical position of the opposing electrode 106.
A sheet guide 105 having a lower arcuate surface extends from the position regulating member 136. An image receiving medium P such as the sheet travels through a space defined between the control electrode 103 and the opposing electrode 106. In this case, various sheets having different thickness to one another can pass through the space since the opposing electrode 106 is biasedly supported by the spring 134. Further, because of the provision of the arcuate guide 105, floating of the sheet P fed to the space can be prevented.
However, since the sheet is brought into contact with the arcuate surface of the sheet guide 105, the sheet may not be smoothly passed through a space between the guide 105 and the opposing electrode 106 if the sheet does not provide sufficient rigidity. If the sheet is excessively flexible, sheet jam may occur at that space.